Production of isoprene



United States Patent 3,238,270 PRODUCTION OF ISOPRENE Byron W. Tnrnquest, Chicago, Ill., assignor to Sinclair Research, Inc., Wilmington, Del., a corporation of Delaware N0 Drawing. Filed Dec. 15, 1961, Ser. No. 159,761 Claims. (Cl. 260-680) The present invention relates to the production of isoprene, more particularly, to the production of isoprene by the thermal pyrolysis of 3,3-dimethyl-a-olefins or 1- halo-3,3-dimethyl alkanes in the presence of hydrogen sulfide.

In view of the similarity of cis-polyisoprene, i.e., synthetic natural rubber, to natural rubber, the demand for isoprene is increasing. The availability and cost of isoprene, however, present formidable barriers to the commercial production of synthetic natural rubber. Thus, there is a continuing search for new sources of isoprene production.

A great deal of research has been directed recently to the production of isoprene by dehydrogenation of C stocks by processes similar to those employed for the production of butadienes. However, these processes have disadvantages which limit their usefulness. Dehydrogenation of isopentane, which is available in relatively large quantities from normal refinery streams and natural gas or field gas streams, is ordinarily carried out by a one-step process such as a process using a chromiumalumina catalyst. In this process, isomerization occurs during dehydrogenation resulting in the formation of one .pound of piperlylene (straight chain C diolefins) for every 2.5 pounds of isoprene in the product.

Furthermore, the quantity of isoamylenes which are available from catalytic cracking or steam cracking of gas oil is quite limited relative to the potential demand for isoprene according to the predicted potential demand for isoprene according to the predicted shortage of 600,- 000 long tons of natural rubber by 1965. Moreover, none of the special catalytic methods is capable of effecting dehydrogenation to isoprene in a desirably selective manner. In all cases, an appreciable amount of the hyrdocarbon feed and/or reaction products is consumed in side reactions such as cracking, polymerization and oxidation.

It has also been reported that the thermal cracking of olefins such as Z-methylpentene-Z affords a relatively simple method for isoprene production. In obtaining maximum selective yields under this process, however, it has been found that the rate of cracking is relatively low.

Low conversion levels are disadvantageous in that they complicate the separation of the isoprene from the crude pyrolyzate stream.

' It has now been discovered that the thermal cracking or pyrolysis of a feed selected from 3,3-dimethyl-a-olefins, and 1-halo-3,3-dimethyl alkanes or mixtures thereof in the presence of hydrogen sulfide results in surprisingly high selective yields of isoprene at high conversion levels with large concentrations of isoprene in the C reaction product. Obtaining large concentrations of isoprene in the C reaction product is particularly important from the viewpoint of isoprene recovery. Due to the close boiling points of the C components, isoprene must often be concentrated by such expensive procedures as extractive distillation or solvent extraction.

The olefin feedstock of this invention has the formula:

CH3 R-( JCH=CH2 wherein R is an alkyl radical of 1 to 6 carbon atoms,

straight or branch chained and with the higher number carbon atom radicals (i.e., of 3 to 6 carbons) the branched structure being preferred. Suitable olefin feeds include, for instance, 3,3-dimethylbutene-1; 3,3-di-methylpentene-l; 3,3-dimethylhexene-1; 3,3,4-trimethyl-pentene- 1; 3,3-dimethyl-5,S-dimethylhexene- 1; etc.

The 3,3-dimethyl-a-olefin feed of the present invention can be prepared by any method known to the art. For instance, 3,3-dimethylbutene-1 may be obtained by a process which comprises dimerization of acetone by aluminum amalgum to form pinacol, rearranging the pinacol to pinacolone by treatment with an acid catalyst, reducing the pinacolone by catalytic methods to the corresponding alcohol, 3,3-dimethylbutanol-2, forming a xanthate or other appropriate ester of the alcohol and subjecting it to thermopyrolysis to obtain 3,3-dirnethylbutene-l. 3,3-dimethylbutene-1 has been obtained by the pyrolysis of the stearic acid ester of 3,3-dimethylbutanol- 2 (see Koch and Van Raay, Greenstoff-Chemie, 32, 161- 174, 1951). The preferred, and more practical process for the production of 3,3-dimethylbutene-1 is disclosed in application Serial No. 94,956 to Emmett H. Burk, Jr., and William D. Hoifman, filed January 11, 1961, and now abandoned. Briefly, the process of the application involves dehydrochlorinatingl chloro 3,3 dimethyl alkane by contact with a solid inert contact material at a temperature of about 450600 C. and a residence time of about .01 to 10 seconds.

The 3,3-dimethyl alkyl halide feedstock of the present invention has the formula:

wherein R is an alkyl radical of 1 to 6 carbon atoms, straight or branch chained and with the higher number carbon atom radicals (i.e., of 3 to 6 carbons), the branched structure being preferred; and X is a halogen such as chlorine, bromine, iodine and fluorine, preferably bromine. The feed materials of the present invention are readily available materials being produced by several methods well known in the art.

1-chloro-3,3-dimethylbutane, for example, can be prepared as described by Lewis Schmerling in the Journal of the American Chemical Society, 67, 115254 (1945). Briefly, the process involves reacting 1 mol of ethylene with t-butyl chloride using a Friedel-Crafts catalyst such as AlCl FeCI BiCl or ZnCl The reaction can take place at atmospheric pressure when employing the reactive Friedel-Crafts catalyst such as AlCl advantageously at a temperature of about -15 C. Under the conditions listed by Schmerling, i.e., complete conversion, the yield of 1-chlor.o-3,3-dimet-hylbutant was reported as theory. Suitable alkyl halide feeds for use in the present invention for instance, l-halo 3,3 -dimethylbutane; 1- halo-3,3-dimethylpentane; 1-halo-3,3 dimethylhexane; 1- halo-3,3-4trimethylpentane; 1-halo-3,3-dimethyl 5,5 dimethylhexane; etc. The alkyl halides can be substituted with non-interfering groups if desired.

In accordance with the process of the present invention the feedstock is subjected to thermal cracking generally at a temperature of about 550 to 900 C., preferably about 650 to 850 C. in the presence of at least about 1 mole percent of hydrogen sulfide up to about 50 or more, preferably about 15 to 30 mole percent based on the olefin or halogen-containing feed. The process can be conducted under vacuum or in the presence of an inert gas diluent such as helium, steam, etc. Operating conditions can be adjusted to give an u-olefin or alkyl halide partial pressure of up to about 0.5 atmosphere, for instance, .05 to 0.5 atmosphere, and a contact time sufficient to produce isoprene, for instance about .001 to 1.0 second. The inert gas, it employed, can be supplied in a molar ratio of at least about 1 mole, for instance, about 1 to 40 or 100 or more moles, preferably to 30 moles per mole of feed. Under preferred conditions the partial pressure is about 0.1 atmosphere and the contact time is about 0.05 to 0.1 second. If desired, the inert gas can be employed as the heating medium to bring the feedstock rapidly to the cracking temperatures. This can conveniently be done by heating the inert gas to a temperature above that desired for conducting the cracking operation, generally at least C. higher than the reaction temperature and not above say 200 C. of the reaction temperature. The inert gas is then quickly mingled with the feed which is at a temperature below that at which any reactions occur. The temperature to which the inert gas is to be heated can be readily determined from the specific heat of the gas, the molar ratios involved, etc. The product from the thermal cracking process is quenched to a temperature of about 500 F. or below and fractionated in ordinary fractionating equipment to obtain isoprene of at least about 95% purity.

The hydrogen sulfide used in the present invention can be hydrogen sulfide per se or it can be sulfur compounds which decompose and/or dissociate under the reaction conditions to produce hydrogen sulfide and whose presence is not otherwise detrimental to the desired reaction. Illustrative of suitable sulfur compounds are elemental sulfur, hydrocarbon sulfides such as methyl-sulfide, tertiary butyl sulfide, carbon disulfide, or inorganic sulfides.

If desired, in addition to the hydrogen sulfide there may be added to the pyrolysis reaction extraneous ethylene and/or isoamylenes and the thermal cracking conducted in their presence as well as that of the hydrogen sulfide. If employed, the amount of ethylene ordinarily used is about 0.5 to 95 mole percent, preferably .5 to 50 mole percent while the amount of isoamylenes that can be used ranges from about .5 to 75 mole percent, preferably .5 to mole percent based on the olefin or alkyl halide feed.

The following examples will serve to illustrate the present invention.

Example I A 12 inch furnace was equipped with a Vycor reactor packed with quartz chips. 3,3-dimethyl-butene-1 containing 19.8 mole percent of hydrogen sulfide was introduced into a stream of steam and the mixture was passed through the reaction zone under the conditions shown in Table I below. After leaving the reaction zone, the gas stream was analyzed. For comparison, a run was also made without the added hydrogen sulfide. The results are shown in Table I below.

TABLE I Isoprene polymer can be eliminated by quicker quenching of the product.

Example 11 3,3-dimethylpentene-1 containing 19.1 mole percent of hydrogen sulfide was thermally cracked in accordance with the general method of Example I and under the conditions shown in Table II below. For comparison, a run was made without hydrogen sulfide addition. The results are shown in Table II below.

* Same as Table I.

The data of Tables I and II show that conducting the thermal pyrolysis of 3,3dimethyl-a-olefins in the presence of hydrogen sulfide greatly improves the yield of isoprene at high conversion levels and provides large concentrations of ioprene in the C reaction product.

Example III 3,3-dimethyl-l-chlorobutane containing 20.2 mole percent H S was thermally cracked in accordance with the general method of Example I and under the conditions shown in Table III below. For comparison, a run was made without hydrogen sulfide addition. The results are shown in Table III.

TAB LE III 20.2 Mole Catalyst None peIrIcent Temperature, F Contact Time. se H O/HC (Mole) Conversion. percent O f-Products, Wt. percent obutylene Other C 3-Methylbutene-1 2-VIethylbutene-1. 2-Methylbuteno-2 Isoprene Polyisoprene 3.3-Dirnethylbutene-1 Vinyl Chloride Others 1 HCl free.

The data of Table III demonstrate that conducting the thermal pyrolysis of lhalo-(3,3-dimethyl) alkanes in the presence of hydrogen sulfide also greatly improved the yield of isoprene at high conversion levels, and provides a large concentration of isoprene in the C reaction product.

I claim:

1. A process for producing isoprene which consists essentially of subjecting to thermal pyrolysis a halogenated hydrocarbon having the structural formula:

wherein R is an alkyl radical of l to 6 carbon atoms and X is a halogen atom, at a temperature of about 550 to 900 C. and a feed partial pressure of up to about 0.5 atmosphere and in the presence of at least 1 mole percent of hydrogen sulfide.

2. The process of claim 1 wherein X is chlorine.

3. The process of claim 1 wherein the hydrogen sulfide is present in an amount of about 1 to 50 mole percent.

4. The process of claim 1 wherein an inert gas in a molar ratio of at least 1 mole of inert gas per mole of feed is employed.

5. A process for producing isoprene which comprises subjecting 3,3-dimethyl-1-chlorobutane to thermal pyrolysis at a temperature of about 650 to 850 C., and a feed partial pressure of up to about 0.5 atmosphere and in the presence of an inert gas in a molar ratio of at least about 1 mole of inert gas per mole of 3,3-dimethy1-lchlorobutane and at least about 1 mole percent of hydrogen sulfide.

6 References Cited by the Examiner UNITED STATES PATENTS 1,925,421 9/1933 Van Peski 260-683 2,415,477 2/ 1947 Folkins et a1. 260683 FOREIGN PATENTS 605,357 10/1961 Belgium. 868,566 5/1961 Great Britain. 916,133 1/1963 Great Britain. 1,251,127 12/1960 France.

PAUL M. COUGHLAN, Primary Examiner.

ALPHONSO D. SULLIVAN, Examiner. 

1. A PROCESS FOR PRODUCING ISOPRENE WHICH CONSISTS ESSENTIALLY OF SUBJECTING TO THERMAL PYROLYSIS A HALOGENATED HYDROCARBON HAVING THE STRUCTURAL FORMULA: 